This invention relates to a new method of fabricating semiconductor devices, and more particularly relates to a novel method of fabricating germanium semiconductor devices and relates to a novel germanium device.
It has been customary to fabricate germanium transistors with a plurality of metal stripes across the top surface thereof. One strip is an emitter which is alloyed into the structure, and the other stripe or stripes are base contacts. These stripes are commonly formed by the so-called "cross-evaporation" technique, involving the placement of a substrate on a heated plate and spacing a metal mask above the plate. The mask has an opening through which the emitter metal and the base contact metal are sequentially evaporated at an angle to the surface of the mask.
The cross-evaporation process, however, has a number of serious drawbacks. One is the matter of cost, since the masks are relatively expensive and the process limits sizes to about six or seven wafers per run. Another problem is that of resolution of the pattern since the masks are spaced from the substrate surface, and the evaporation is at an angle to the wafer surface. These problems create a practical lower limit to the cross-sectional area and spacing of the stripes. The spacing and cross-sectional area, in turn, limit the high frequency response of the resulting device.
An object of the present invention is to provide a method of fabricating a germanium semiconductor device which provides a high degree of freedom of device geometry.
Another object of the invention is to provide a method of fabricating a germanium semiconductor device which permits closer spacing between an alloyed emitter and a base contact.
An additional object of the invention is to provide a germanium semiconductor device which has a high degree of resolution of the metal portions on the surface.
A further object of the invention is to provide a germanium mesa transistor having higher frequency response and lower noise level than previously attainable.
A feature of the invention is a method of fabricating a germanium semiconductor device using a photosensitive material in which substantially all particles therein are of a size less than about one micron.
Another feature of the invention is a novel method of fabricating a semiconductor device having improved definition of metal geometry achieved through the use of the above photosensitive material in combination with an etchant for a gold-containing layer comprising an alkali metal compound and an organic acid derivative.